The present invention relates to a method and system for storage of a gas, such as natural gas, by adsorption, and to a gas occluding material based on adsorption and a process for its production.
An important issue in the storage of a gas, such as natural gas, is how gas which is at low density under normal temperature and pressure can be efficiently stored at high density. Even among natural gas components, butane and similar gases can be liquefied at normal pressure by pressurization at a relatively low pressure (CNG), but methane and similar gases are not easily liquefied by pressure at normal temperature.
One method that has conventionally been used as a method for storage of such gases which are difficult to liquefy by pressure at near normal temperature, is liquefaction while maintaining cryogenic temperature, as in the case of LNG and the like. With this type of gas liquefaction system it is possible to store a 600-fold volume at normal temperature and pressure. However, in the case of LNG for example, a cryogenic temperature of xe2x88x92163xc2x0 C. or below must be maintained, inevitably leading to higher equipment and operating costs.
An alternative being studied is a method of storing gas by adsorption (ANG: adsorbed natural gas) without special pressure or cryogenic temperature.
In Japanese Examined Patent Publication No. 9-210295 there is proposed an adsorption storage method for gas such as methane and ethane in a porous material such as activated carbon at near normal temperature, in the presence of a host compound such as water, and this publication explains that large-volume gas storage is possible by a synergistic effect of the adsorption power and pseudo-high-pressure effect of the porous material and formation of inclusion compounds with the host compound.
However, even this proposed method is not able to realize storage density comparable to that of storage methods using cryogenic temperature, such as with LNG.
The use of activated carbon has been proposed as a gas occluding material for storage of gases that do not liquefy at relatively low pressures of up to about 10 atmospheres, such as hydrogen and natural gas (see Japanese Unexamined Patent Publication No. 9-86912, for example). Activated carbon can be coconut shell-based, fiber-based, coal-based, etc., but these have had a problem of inferior storage efficiency (storage gas volume per unit volume of storage vessel) compared to conventional gas storage methods such as compressed natural gas (CNG) and liquefied natural gas (LNG). This is because only pores of a limited size effectively function as adsorption sites among the various pore sizes of the activated carbon. For example, methane is adsorbed only in micropores (2 nm or less), while pores of other sizes (mesopores: approximately 2-50 nm, macropores: 50 nm and greater) contribute little to methane adsorption.
It is a first object of the present invention to provide a gas storage method and system that can accomplish very high storage density by adsorption without using cryogenic temperatures.
It is a second object of the invention to provide a gas occluding material with higher storage efficiency than activated carbon.
According to the first aspect of the invention for the purpose of achieving the aforementioned first object, there is provided a gas storage method comprising
keeping a gas to be stored and an adsorbent in a vessel at a low temperature below the liquefaction temperature of the gas to be stored so that the gas to be stored is adsorbed onto the adsorbent in a liquefied state,
introducing into the vessel kept at the low temperature a gaseous or liquid medium with a freezing temperature that is higher than the above-mentioned liquefaction temperature of the gas to be stored, for freezing of the medium, so that the gas to be stored which has been adsorbed onto the adsorbent in a liquefied state is encapsulated by the medium which has been frozen, and
keeping the vessel at a temperature higher than the liquefaction temperature and below the freezing temperature.
According to the first aspect of the invention there is further provided a gas storage system characterized by comprising
a gas supply source which supplies gaseous or liquefied gas,
a gas storage vessel,
an adsorbent housed in the vessel,
means for keeping the contents of the vessel at a low temperature below the liquefaction temperature of the gas,
a gaseous or liquid medium with a freezing temperature which is higher than the liquefaction temperature of the gas,
means for keeping the contents of the vessel at a temperature higher than the liquefaction temperature and lower than the freezing temperature,
means for introducing the gas from the gas supply source into the vessel and
means for introducing the medium into the vessel.
According to the first aspect of the invention there is further provided a vehicle liquefied fuel gas storage system characterized by comprising:
a liquid fuel gas supply station,
a fuel gas storage vessel mounted in the vehicle,
an adsorbent housed in the vessel,
means for keeping the contents of the vessel at a low temperature below the liquefaction temperature of the gas,
a gaseous or liquid medium with a freezing temperature which is higher than the liquefaction temperature of the fuel gas,
means for keeping the contents of the vessel at a temperature higher than the liquefaction temperature and lower than the freezing temperature,
means for introducing the fuel gas from the fuel gas supply station into the vessel and
means for introducing the medium into the vessel.
According to the second aspect of the invention for the purpose of achieving the aforementioned second object, there is provided a gas occluding material comprising either or both planar molecules and cyclic molecules. It may also include globular molecules.
In the gas occluding material of the invention, the gas is adsorbed between the planes of the planar molecules or in the rings of the cyclic molecules. It is appropriate for the ring size of the cyclic molecules to be somewhat larger than the size of the gas molecules.